2-42
Q43. How does ftc affect receiver gain, if at all?
SPECIAL RECEIVERS
The basic receiver of a radar system often does not meet all the requirements of the radar system, nor
does it always function very well in unfavorable environments. Several special receivers have been
developed to enhance target detection in unfavorable environments or to meet the requirements of special
transmission or scanning methods. A radar system with a moving target indicator (mti) system or a
monopulse scanning system requires a special type of receiver. Other types of special receivers, such as
the logarithmic receiver, have been developed to enhance reception during unfavorable conditions. These
receivers will be discussed in general terms in this section.
Moving Target Indicator (mti) System
The MOVING TARGET INDICATOR (mti) system effectively cancels CLUTTER (caused by fixed
unwanted echoes) and displays only moving target signals. Clutter is the appearance on a radar indicator
of confusing, unwanted echoes which interfere with the clear display of desired echoes. Clutter is the
result of echoes from land, water, weather, and so forth. The unwanted echoes can consist of GROUND
CLUTTER (echoes from surrounding land masses), SEA CLUTTER (echoes from the irregular surface of
the sea), or echoes from the clouds and rain. The problem is to find the desired echo in the midst of the
clutter. To do this, the mti system must be able to distinguish between fixed and moving targets and then
must eliminate only the fixed targets. This is accomplished by phase detection and pulse-to-pulse
comparison.
Target echo signals from stationary objects have the same phase relationship from one receiving
period to the next. Moving objects produce echo signals that have a different phase relationship from one
receiving period to the next. This principle allows the mti system to discriminate between fixed and
moving targets.
Signals received from each transmitted pulse are delayed for a period of time exactly equal to the
pulse-repetition time. The delayed signals are then combined with the signals received from the next
transmitted pulse. This is accomplished in such a manner that the amplitudes subtract from each other as
shown in figure 2-30, views Aand B. Since the fixed targets have approximately the same amplitude on
each successive pulse, they will be eliminated. The moving target signals, however, are of different
amplitudes on each successive pulse and, therefore, do not cancel. The resulting signal is then amplified
and presented on the indicators.
